Rackwarehouse . com and search thule roof rails, they make them about 60 inches in length, you can mount them right to the top of the roof using the included silicone sealant and screws. Select the Thule TK1 Tracker kit, the feet and bars, and whatever accessory you want, whether it be a cargo box, kayak racks, bike racks, skis etc.
I used to own a Viking popup and mounted these exact rails/racks and carried 3 kayaks on the roof, or 5 bikes on the roof, or a combination of both, it worked really well and I never developed leaks or problems. Best thing, my kayaks sat below the level of my tow vehicle roof (suburban at the time)
Here is a link http://www.rackwarehouse.com/thule-t...il-tracks.html